Following from the observations that post-addicts and non-opioid users are differentially sensitive to opioids, and perhaps even respond qualitatively differently, and the possibility that such differences either predispose certain persons to opioid abuse and/or contribute to relapse, this study was conducted to experimentally examine such population differences in response to mu and kappa opioids. Prominent measures included discrimination thresholds of behavioral effects, physiologic responses, and neuroendocrine response. Post-addict and opioid-naive subjects were intended to be separately tested for comparison. Initial phase testing involving post-addict volunteers was completed, however, changes in priorities resulted in the termintion of the protocol before opioid-naive subjects were tested.